FIG. 1 illustrates the blade opening of a prior art retractable rule assembly wherein the housing is formed of a plastics material. The rule assembly is generally indicated at 300 and the housing is generally indicated at 302. The housing 302 includes a blade opening 304 through which an elongated metal blade extends. The blade is not shown for purposes of clearly showing the blade opening 304. A spring-driven reel 306 is mounted in the housing 302 and the blade is wound thereon with its extended portion extending through the blade opening 304. During use of the rule assembly, the blade is extended from the reel 306 through the blade opening 304 for purposes of taking measurements, and then is retracted automatically by the spring driving the reel 306. During the repeated usage over time, the lateral side edges of the blade can come into contact with the lateral side edges of the opening 304, particularly during retraction by the spring. Because the spring retracts the blade at a relatively high speed, and the blade is formed from a metal, which is significantly harder than the plastics material of the housing 302, the blade's lateral side edges can wear grooves 308 into the plastics material at the lateral side edges of the opening 304.
As shown in FIG. 1, these grooves can become relatively deep over time. Specifically, the blade initially forms a slight groove during its initial usage, and over time the blade tends to “find” this groove and repeatedly wear the same groove to increased depth, rather than uniformly wearing the entire height of the lateral sides edge. As an analogy, this can be compared with the manner in which one cuts lumber with a saw—first the user moves the saw over the edge of the uncut lumber to form an initial groove, and then can cut freely with more power because the initial groove will guide the saw as the cutting depth increases.
This formation of grooves 308 can be problematic for a number of reasons. For example, as the blade rides in a formed groove 308, the measuring indicia on the blade edges can be worn off by the upper surface of the grooves, which makes it difficult to take proper measurements. Also, as the blade rides in the groove, friction is created, which provides additional resistance to blade retraction. To offset this additional resistance, a larger more powerful spring may be needed, which adds cost. In addition, these grooves 308 can create “fulcrum points” about which the extended portion of the blade can bend sharply during retraction. Specifically, in some rule assemblies the blade opening is designed to be vertically higher than the blade itself to accommodate some vertical movement of the blade to minimize the chances of sharp bends in the blade as it retracts. However, when the blade rides in a groove 308, the groove 308 prevents such vertical movement of the blade, and thus can create a point about which sharp bends occur. Sharp bending of the blade is undesirable, because over time it can lead to blade fatigue and failure.